Compressed air supply installations are used in vehicles of all types, in particular to supply an air spring system of a vehicle with compressed air. Air spring systems may also include ride height adjustment devices with which the distance between the vehicle axle and the vehicle chassis can be adjusted. An air spring installation of a pneumatic system cited initially comprises a number of air bellows pneumatically connected to a common line (gallery) which are able to raise the vehicle chassis when increasingly inflated, and lower it again when the fill level decreases. As the distance between the vehicle axle and vehicle chassis (or the ground clearance) increases, the spring travels become longer and even greater bumps in the ground can be overcome without contact with the vehicle chassis. Such systems are used in off-road vehicles and Sport Utility Vehicles (SUV). In particular in SUVs with very powerful engines, it is desirable to provide the vehicle firstly with comparatively low ground clearance for high speeds on the road, and secondly comparatively large ground clearance for off-road use. It is furthermore desirable to be able to implement a change in ground clearance as quickly as possible, which increases the requirements with regard to speed, flexibility and reliability of a compressed air supply installation.
A compressed air supply installation for use in a pneumatic system with a pneumatic installation, for example an air spring installation described above, is operated with compressed air from a compressed air feed, for example in a pressure level range from 5 to 20 bar. The compressed air is made available to the compressed air feed using an air compressor. The compressed air feed is pneumatically connected to a compressed air connection for supplying the pneumatic installation, and secondly pneumatically connected to a purge connection. The compressed air supply installation may be purged to the purge connection by the release of air via a purge valve assembly.
To ensure long-term operation of the compressed air supply installation, this comprises an air dryer with which the compressed air is dried. This avoids the accumulation of moisture in the pneumatic system, which at comparatively low temperatures can lead to crystallization damaging the valve, and can also lead to undesirable defects in the compressed air supply installation and in the pneumatic installation. An air dryer comprises a drying agent, normally granulate through which the compressed air can flow, so that at comparatively high pressure, the granulate can absorb the moisture contained in the compressed air by adsorption. An air dryer may also be configured as a regenerative air dryer. This may be achieved in that on each purge cycle—at comparatively low pressure—the dried compressed air from the air spring system flows through the granulate in counterflow or in co-flow relative to the filling direction. For this, the purge valve arrangement can be opened. For such a use—also called a pressure-change adsorption—it has proved desirable to configure a compressed air supply installation flexibly and at the same time reliably, in particular to allow a comparatively rapid purge with a pressure change which is nonetheless adequate for regeneration of the air dryer.
DE 35 429 74 A1 by the applicant describes a compressed air supply installation cited initially for a ride height control device for vehicles with air springs, with which, depending on the vehicle load, a predefined distance of the vehicle passenger cell from the vehicle axle can be maintained by filling and evacuating the air springs. The device contains a normally closed magnetic purge valve and a safety valve which can be controlled using the pressure in the air springs. Such a compressed air supply installation can still be improved.
A compressed air supply installation cited initially is also disclosed in EP 1 165 333 B2 in the context of a pneumatic system cited initially with an air spring installation. As well as a main purge line which can be shut off separately, this comprises a high-pressure purge line which has an additional high-pressure purge valve, as well as the main purge valve pneumatically controlled with a control valve in the main purge line, which is connected in parallel to the main purge line. The free flow cross-section of the separate high-pressure purge valve is smaller than that of the main purge valve. Such a compressed air supply installation can still be improved. It has been found that on purging such a compressed air supply installation via the high-pressure purge line, dry air is purged which is not used for regeneration of the drying agent. This equates to an unnecessary waste of dry air, in particular for the case that a flexible, fast and yet reliable actuation of the compressed air supply installation is required which is suitable for the above applications, with correspondingly high actuation rate.
All the above-mentioned solutions with a purge valve assembly in the form of a controllable magnetic valve assembly provide that, in the non-activated state of the magnetic part, the pneumatic part of the magnetic valve assembly is closed, i.e. the solutions provide a normally closed magnetic valve assembly. In particular with regard to the solution disclosed in DE 35 42 974 A1, it has been found that a normally closed magnetic purge valve assembly may be disadvantageous since, frequently, an additional pressure-limiting or safety valve must be provided in order to guarantee a reliable function. Since, in normally closed state, a relay valve of the magnetic valve assembly is closed, in individual cases this can lead to a valve body sticking on the valve seat, and in some cases the switch function of the relay valve of the magnetic valve assembly is not reliably guaranteed.
DE 39 19 438 C2 discloses a compressed air supply installation in which the air dryer can be connected to an outlet to atmosphere via an electrically actuatable valve device. The valve device has an electric control inlet which is connected to an electronic control device via an electric line with an electric control outlet; in non-activated state, the valve device is open. To inflate an air spring, the valve device assumes a switch position in which the connection of the air dryer to the outlet to atmosphere is interrupted.
DE 197 242 747 C1 discloses a ride height control device for vehicles with air springs, wherein a pneumatically controllable directional valve is provided in the pneumatic main line for switching via a control line, with an air control pressure generated by the compressor. The control line may be connected to atmosphere via a further directional valve, so that when the compressor is not running, the directional valve in the pneumatic main line can change from the one switch state which connects the compressor to the air dryer, to the other switch state which connects the air dryer to atmosphere.